The invention relates to an antenna comprising a radiating surface and a base surface.
Strip antennas, also called patch antennas, are characterized by a low weight and a small cross-section, which results in an easy handling and a wide field of application.
Known strip antennas consist of a metal strip which is arranged at a definable distance parallel to a metallic base surface. A homogeneous dielectric is usually situated between the strip and the base surface. The length of the metal strip is selected such that the electric length of the line which the strip forms together with the base surface is approximately half a wavelength (in the dielectric) long. The width of the metal surface essentially defines the impedance of the antenna; the distance of the strip from the base surface essentially determines the bandwidth. This distance is simultaneously the overall height of the strip antenna. Normally, the overall height is between one twentieth and one fifth of the free-space wavelength at band center, a greater overall height resulting in a higher bandwidth.
One disadvantage of the strip antennas is the small bandwidth. For enlarging the bandwidth, for example, the shape of the metal strip is selected such that the resonance frequencies of two or more oscillation modes of the antenna have a relatively small frequency spacing. As a result, bandwidth ratios of up to 1.6:1 can be reached. The bandwidth ratio is defined as the ratio of the upper frequency limit to the lower frequency limit. Such strip antennas are known, for example, from European Patent Document EP 0 939 628 B1 and International Patent Document WO 2004/021514 A1.
European Patent Document EP 0 989 628 B1 provides that the base surface is connected with the radiating surface by means of a coaxial cable, the coaxial cable being used for guiding signals to the radiating surface. In this case, the base surface has a vertical edge which extends in a vertical manner from the base surface, so that an “L”-shaped or “U”-shaped cross-section is obtained. One disadvantage of this arrangement is that the bandwidth is too small for certain fields of application.
For certain commercial and military fields of application such as hopping operations for military communication services, battlefield monitoring systems, transmission systems where several transmitters operating at different frequencies are simultaneously connected to the same antenna, and for corresponding receiving systems, antennas are required which, although they have a low overall height and a small size, have a considerably larger bandwidth than can be achieved by means of strip antennas. There are naturally other types of antennas which have the required bandwidth ratio. However, in many cases, these have significantly larger dimensions.
It is therefore an object of the invention to provide an improved antenna by means of which the bandwidth can be increased significantly.
The antenna according to the invention has one or more discrete components arranged between the radiating surface and the base surface, the radiating surface having a tapering with respect to its width B and with respect to its height H to the base surface. The term “tapering” means in this case that, along the longitudinal dimension L of the radiating surface, the width B as well as the height H of the radiating surface vary over the base surface.
The radiating surface advantageously has a maximal length Lmax≦0.6 λmax, a maximal width Bmax≦λmax, and a maximal height Hmax≦0.4 λmax with respect to the base surface, λmax being the free-space wavelength at the lower frequency limit fu of the frequency band of the antenna. For the standing wave ratio VSWR, in a frequency range [fu, fo] with fu and fo as the lower and upper frequency limit of the frequency band of the antenna, preferably VSWR≦3 applies, for the bandwidth, fo/fu≧1.4.
The radiating surface advantageously has a constant tapering. In this case, the radiating surface has the shape of an isosceles triangle. The radiating surface together with the base surface forms a TEM waveguide with a constant characteristic wave impedance.
The devices for feeding electromagnetic energy to the antenna are preferably arranged in the area of the smallest distance between the radiating surface and the base surface. For a triangular radiating surface, this can expediently be a corner of the radiating surface.
The feeding preferably is a coaxial feeding. The coaxial internal conductor is physically connected with the radiating surface, while the external conductor is physically connected with the base surface of the antenna. The tapering of the width of the radiating surface and of the height of the radiating surface over the base surface is expediently selected to fit the impedance of the connected feeding cable, because then the higher oscillation modes of the antenna occurring at the feeding point are excited only with a low amplitude.
The discrete components, which are distributed below the radiating surface at predefinable sites with predefinable values, are used for improving the adaptation for the lower part of the frequency range. Values and sites can be selected corresponding to the respective demands on the adaptation and on the radiation diagram of the antenna. The discrete components may particularly be inductances and/or capacitances.
However, shapes other than triangular shapes and non-constant height and width tapering of the radiating surface of the antenna also make sense in special cases. As a result, further improvements of the adaptation and the shape of the radiation diagram are conceivable.
The term “discrete component” is understood in the functional sense. Instead of a discrete inductance or capacitance, an implementation as a conduction printed on a substrate (not shown) can also be used.
The antenna according to the invention permits a very broadband radio operation, such as a hopping operation. In addition, a simultaneous feeding of the antenna by means of several transmission lines, which are distributed in a wide frequency range, is conceivable. Furthermore, it becomes possible by means of the antenna according to the invention to simultaneously receive several received signals situated in a broad frequency band.
Another advantage of the antenna according to the invention is the possibility of using this broadband antenna directly in front of a metallic or non-metallic wall without an impairment of its adaptation or its radiation diagram. This can also be achieved in the case of a conformal adaptation of the radiating surface to a possibly curved shape of the metallic wall. In the case of a metallic wall, the wall itself can be used as a base surface. The wall could, for example, be part of the surface of a vehicle, a ship or an airplane. As a result of the low overall height of the antenna, the antenna projects only slightly beyond the vehicle surface. This applies to implementations for the VHF range, the UHF range and naturally for the microwave range.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The invention as well as additional advantageous embodiments of the invention will be explained in detail in the following by means of drawings.